remove privateKey references from at_lookup_impl and enforce at_chops

[murali-shris]

Describe the bug

After at_chops and at_auth package implementation, private key should no longer be read from preferences and used in code.
Some code was left behind in atLookUp as a fallback mechanism if atChops is not set.
Changes to enforce atChops and remove privateKey from test files has been done in
atsign-foundation/at_client_sdk#1142

Steps to reproduce

n/a

Expected behavior

• Remove call to authenticate method in AtLookupImpl --> _process
  else if (privateKey != null) {
  logger.finer('calling pkam without atchops');
  await authenticate(privateKey);
  }
• Test the changes with at_client/at_client_mobile/at_onboarding_cli branches.
• Do a sanity with mobile/CLI apps

Screenshots

No response

Smartphones

No response

Were you using an atApplication when the bug was found?

No response

Additional context

No response

[purnimavenkatasubbu]

Made the change and raised a draft PR and tested with all the apps and the cli apps. Moving it to next sprint for PR review and for publishing the package

[purnimavenkatasubbu]

at_lookup:3.0.42 has been published to pub.dev

[murali-shris]

Reopening the issue since it is a breaking change.
Change has to be done as part of major version release.